A gas shielded arc welding, which is a non-consumable electrode type and is referred to GTAW (Gas Tungsten Arc welding), is conventionally used in welding a structure (workpiece) made of metal and non-ferrous base material. The gas shielded arc welding includes TIG welding (Tungsten Inert Gas welding) or plasma arc welding.
TIG welding is performed while forming a welding pool by melting a workpiece with arc heat which is generated between a non-consumable electrode (−) and the workpiece (+) using a TIG welding torch including a torch nozzle and a non-consumable electrode. Also, during the welding, a shielding gas released from a torch nozzle which surrounds the electrode blocks atmosphere (air).
In contrast, regarding the plasma arc welding, electrically-gerenrated plasma gas (also referred to as working gas) flows between a non-consumable electrode and an insert chip, using a plasma arc torch including the non-consumable electrode, the water-cooled insert chip (also referred to as a constrained nozzle) and a shield cap. A plasma arc with improved energy density is generated by concentrating the generated plasma flow (plasma jet) by using the insert chip, and then using wall effect (effect of stabilizing the flow of the air flow of the plasma flow) due to the inner wall of the shape of the insert chip and thermal pinch effect (effect of becoming a high temperature due to constrictation by cooling the surroundings of the plasma flow) obtained by cooling the insert chip. The plasma arc is further narrowed down by receiving the thermal pinch effect of shielding gas emitted from the shield cap.
The plasma arc welding has high energy density and is performed using a plasma arc, which has a narrowed cylindrical arc shape, as a heat source. In addition, the plasma arc has a transferred type and a non-transferred type. The transferred plasma arc can only be applied to a conductive workpiece becase electric current flows between the non-consumable electrode (−) and the workpiece (+). On the other hand, the non-transferred plasma arc can be used even for non-conductive workpiece applications because electric current flows between the non-consumable electrode (−) and the insert chip (+). Furthermore, the plasma arc is not limited to welding applications described above, for example, it is also used for brazing, joint, cutting, thermal-spraying, melting furnace or the like, against the workpiece.
The plasma arc torch and the power unit for the plasma arc system are generally more expensive than the torch and power unit for TIG welding system. On the other hand, a torch used in both TIG welding torch and plasma arc torch by replacing the spare parts attached to the plasma arc torch body, has been proposed (e.g., see Patent Document 1).
However, in the technique described in Patent Document 1, it is necessary to provide different replaceable parts if being used as a TIG welding torch or as a plasma arc torch. Further, a relatively expensive power unit is used for the plasma arc.
Conventionally, as a non-consumable electrode welding process of welding a workpiece (base material), TIG welding or a plasma welding method is used. Since the heat concentration is excellent, it is possible to perform the welding process at a high welding speed and to obtain a result with a narrow bead width and a small distortion, when the plasma welding method is used, compared with the TIG welding method.
The plasma welding method is a plasma arc method (transferred type plasma), a plasma jet method (non-transferred type plasma), or the like.
In general, a welding system used with the plasma arc method (welding machine of the plasma arc system) includes a torch; a main power unit in which a negative terminal is connected to an electrode constituting the torch, and a positive terminal is electrically connected to a workpiece; a pilot arc power unit which is electrically connected to a main power unit via a wire; a change-over switch which includes wires for connecting the pilot arc power unit with a insert chip (also referred to as “restraint nozzle”) which constitutes the torch; and a high-frequency device for generating a pilot arc.
When using a plasma welding system of the plasma arc system, it is possible to perform relatively deeper-penetration welding.
When using a plasma jet type plasma welding system (plasma jet type plasma welding machine), an electrode constituting a torch is connected to a negative terminal of a main power unit, a insert chip constituting the torch is connected to the main power unit at a positive terminal.
In the plasma jet method, since no current flows through the workpiece, it can also be used as a heat source for thermal spraying or a heat source of furnace.
A complex type plasma welding system disclosed in Patent Document 2 includes a torch, a power unit (hereinafter, referred to as “plasma power unit for a complex system”). The plasma power unit for the complex system includes a main arc power unit (also referred to as “main power unit”), a pilot arc power unit, and a high frequency device.
A negative terminal of the main arc power unit (main power unit) is connected to an electrode which constitutes a torch, and a positve terminal is electrically connected to a workpiece. The pilot arc power unit is electrically connected to the main arc power unit and an insert chip.
When the plasma welding system of complex mode above is used, since it is possible to obtain stable plasma even at very low current, it is possible to weld an ultra thin sheet which is difficult to be welded by using the TIG welding method.
By the way, the power unit (hereinafter referred to as “TIG power unit”) constituting the TIG welding system (TIG welding machine) uses a welding power unit and a high-frequency device, which is a very simple structure.
Thus, as compared with the TIG power unit, since the plasma power unit is expensive, it is a factor of increasing the cost of the plasma welding system.
For this reason, even if the plasma welding system has a better welding performance, there is a problem that it is difficult to adopt a plasma welding system in which it is possible to obtain deep penetration, due to the disadvantage that the initial investment cost is high.
[Patent Document 1] Japanese Utility Model Publication No. 1981-126981
[Patent Document 2] Japanese Patent Publication No.1988-194867